As cards, tickets, brands of commodities and important documents relate to public security and/or important economic interests, new technologies, new materials and new concepts should be applied in this field to keep the anti-counterfeiting technology ahead of that of counterfeiters. Also, because cards, tickets, commodities are primarily circulated in the public places, one of the main requirements of the optical anti-counterfeiting technology is easy to be identified by the public. Optical variable images based on laser holographic technology is used for security identification on cards, passports, money, bonds, etc., for its obvious visual effect. However, with the popularity of laser holographic technology, the security effect of normal optical variable images gradually reduces.
Micro-optical elements based on Moore amplification principle can amplify micron-scale images in hundreds of times to form millimeter-scale visible images. In the mean time, the presented image will change with the changing of viewing angle, to form a dynamic effect. So it is particularly concerned by the financial field in recent years.
U.S. patents and patent applications U.S. Pat. No. 5,712,731, US2005/0180020A1 and US2008/0037131A1 disclosed to combine micro lens array with micro-graphics array to apply to a security device, wherein the micro-graphics array was made by gravure printing, with a minimum resolution of 5 microns. The main producing method is, coating on flexible films, such as PET, with photoresist, imprinting grooves of several microns depth on the surface of the photoresist using relief plate with micro graphics, filling the grooves with ink by pasting and scratching to let the micro graphics show relative color. In this method of making micro graphics, the color of image is obtained by filling colored ink in the grooves, so the color is mono-color and cannot get colored images; the characteristic size of micro graphics is several microns, while the size of particles of common ink is tens of microns, so, it is required to use special ink with nano-scale particles as the filling color ink; further, in order to let enough ink to be filled into grooves, so as to enhance the contrast between the micro graphics and the background, the groove depth should be deeper than 3 microns, thus, metal imprint templates with large aspect ratio is needed, which resulting in an increase in the difficulty of the process, especially when making large-scale dynamic images.
Another method to make color micro images is with traditional printing way. Luscher's top imaging system JetScreen2000 for large-scale screen and offset printing, for example, has a printhead with 256 nozzles, providing 700 dpi resolution image output; Amanda, a subsidiary of Autoprint Srl., launches a new thermal inkjet printer DigitAll, which can achieve a resolution of 720×720 dpi image output; as for digital screen printing, Scitex's new super wide format printer Grandjet has a resolution of 370×370 dpi; Nur's Fresco printer can achieve a resolution of 720 dpi.
Thus, at the current level of printing technology, 1-2 μm resolution micro graphics cannot be printed, so as not to have the accuracy for Moore amplification. Meanwhile, inefficient printing method can hardly achieve large volume, low cost production of micro graphics.
Chinese Patent CN101850680A discloses a security film with dynamic three-dimensional effect, achieved by random distribution of the center coordinates of each micro lens in the micro lens array layer, a micro lens in the micro lens array layer is set correspondence to a micro graphic in the micro graphic layer. Wherein noted, each micro-graphic can be consisted with micro-nano structures, produced by UV imprint directly implemented on the organic thin film material.
However, financial security products usually required to be observed in ordinary natural light, therefore, easily identifiable visual feature is needed, such as, the dynamic images and pattern background should have high contrast, or the images should have color or grayscale diversification, or also have directed display effect. The micro graphics made by normal nanostructures in CN101850680A, can only add background color through printing, which affects the contrast, so as to be difficult to satisfy the above requirements.